Human serum albumin (HSA), one of the proteins measurable in urine, has clinical significance for detecting the early stages of nephropathy, i.e. an abnormal state of the kidney. A high percentage of individuals suffering from insulin dependent diabetes mellitus (IDDM) and non-insulin dependent diabetes mellitus (NIDDM) eventually excrete HSA at levels above the upper end of the normal population. This stage of "microalbuminuria" becomes progressively worse and leads to nephropathy. Since the kidney damage at the stage of microalbuminuria can be controlled or reversed by administering appropriate therapy, it is well recognized that measuring microalbuminuria is part of the comprehensive care of IDDM and NIDDM.
Other urine born proteins, e.g. IgG, alpha-1-microglobulin, Bence-Jones protein and N-acetyl-b-D-glucosaminidase, are useful as markers to detect and differentiate prerenal, glomerular and postrenal forms of proteinuria. Proteinuria, which is indicated if the protein concentration in urine is greater than 30 mg/dL, is the common symptom for a variety of different kidney diseases. Accordingly, there is a need on the part of nephrologists, diabetologists and cardiologists for test methods that are sensitive, specific and quantitative for the determination of these proteins in urine.
In order to increase the sensitivity and specificity of urinary protein assays and minimize the problem of high urine flow rate resulting in urine dilution, protein/creatinine ratios are used in urine protein assay results to normalize the urine concentration. Typical urinary protein analyses involve immunoassays such as radioimmunoassay, enzyme immunoassay, latex assisted immunoassay and immunoturbidimetric assay. Common creatinine assays, such as the alkaline Jaffe and Benedict-Behre methods, are run at a high pH, typically in the range of from 11.5 to 12.5. The common practice in present day clinical laboratories is to run the protein and creatinine assays separately, and then combine the values obtained from these assays to generate the protein creatinine ratio. Since patients with high urine flow rates would have artificially low protein values because of the urine's dilution and since creatinine is a good marker for the dilution of urine, using the protein/creatinine ratio, therefore, eliminates the problem of urine dilution and gives a more accurate reflection of the true protein excretion rate.
In Clin. Chem. 32/8, 1544-1548 (1986); Watts et al discuss four immunochemical methods (radioimmunoassay, radial immunodiffusion, immunoturbidimetry and enzyme-linked linked immunosorbent assay) for measuring urinary albumin at low concentrations. They point out that diabetic nephropathy is a major cause of death in insulin dependent diabetics and that the clinical chemistry laboratory requires a technique that is sensitive, specific for albumin and practicable. Watts et al go on to point out in Practical Diabetes Vol. 9, No. 3, Pp. 84-86 that microalbuminuria is a supranormal quantity of urinary albumin which escapes detection Dy indicator dye binding tests and that it is a powerful marker of both early nephropathy and cardiovascular disease in patients with diabetes mellitus.